Onset Of Continental Collision Research Articles

Timing of India-Asia collision and significant coupling between them around 51 Ma: Insights from the activation history of the Zhongba-Gyangze thrust in southern Tibet

Abstract Constraining the timing of tectonic coupling between converging plates is crucial for understanding the transition from continental subduction to continental collision. In the case of the India-Asia collision, thrusting of an accretionary complex onto the Indian continental margin provides the most direct temporal constraint on the early stages of continental collision, as it represents the most immediate upper-crustal fault system corresponding to plate coupling. Here, we used structural analysis combined with K-Ar dating and hydrogen isotopes of authigenic illite and muscovite to unravel the time-progressive development of the Zhongba-Gyangze thrust (ZGT), which represents a tectonic boundary fault in southern Tibet. Our results suggest that the ZGT evolved from its initiation as a single fault zone infiltrated by metamorphic fluids with high δD values (–47‰ to –55‰) at ca. 80 Ma to multiple deformation localization zones starting around 51 Ma. This latter phase is represented by the development of different generations of authigenic 1 M/1Md illite and significant input of meteoric fluids with δD values ranging from –71‰ to –98‰ through multiple episodes of brittle fault reactivation. A Late Cretaceous tectono-thermal event related to the subduction of a Neotethyan oceanic ridge may have been responsible for the formation of 2M1 illite/muscovite at ca. 80 Ma. The oldest (ca. 51 Ma) 1 M/1Md illite age coincides with the first major pulse of shortening in the upper plate after the initial India-Asia contact. Given the synchronous deceleration of India-Asia convergence, the ca. 51 Ma deformation pulse across the Yarlung-Zangbo suture zone demarcates strong coupling (i.e., the onset of continental collision) between India and Asia at this time.

Northward subduction of the South Qilian ocean: Insights from early Paleozoic magmatism in the South-Central Qilian belts

The Qilian orogen consists of the North Qilian belt, the Central Qilian block and the South Qilian belt and is regarded as part of the northern margin of the Proto-Tethys Ocean. The tectonic mechanism responsible for the early Paleozoic magma generation in the Qilian orogen is crucial to understanding the ocean-continent configurations and the closure of the Proto-Tethys Ocean. This paper compiles the early Paleozoic mafic and granitic intrusions in the South Qilian belt and the Central Qilian block, to constrain the subduction and accretion of the South Qilian ocean. The early Paleozoic mafic intrusions in the Central Qilian block indicate that the influence of slab-derived fluids dominates their petrogenesis, while the mafic intrusions in the South Qilian belt were mostly affected by involvement of significant sediment-derived melts in their formation. They are supportive of a mélange model, in which melting of mélange diapirs originating at the slab-mantle interface generates fluid-metasomatised magma source in the deep mantle under the Central Qilian block and sediment-derived melts-metasomatised magma source under the Hualong block in the South Qilian belt, consistent with northward subduction of the South Qilian ocean. It is proposed that northward intra-oceanic subduction of the South Qilian Ocean occurred at 530–470 Ma in front of the Hualong block and continued at 470–446 Ma but was then accompanied by the development of a mature volcanic arc. Continuous subduction resulted in accretion of the Hualong block northwards onto the Central Qilian block, and this collision started at ca. 446 Ma and lasted to ca. 420 Ma. The final closure of the South Qilian ocean was marked by the onset of continental collision between the Qaidam-Quanji block and the amalgamated Hualong-Central Qilian block at ca. 440 Ma, and also the cause of the A-type granitoids after final collision occurred at ca. 420 Ma.

Formation of the Alpine Orogen by Amagmatic Convergence and Assembly of Previously Rifted Lithosphere

The tectonic and magmatic characteristics of the Alps and Pyrenees during convergence are quite distinct from characteristics associated with classic Benioff-type oceanic subduction. From the initiation of subduction at passive margins until the onset of continental collision, the closure of the Western Tethys never produced a long-lived magmatic arc. This is a consequence of the 3-D architecture of the Western Tethys (a series of hyper-thinned basins and continental blocks) and its narrow width (<500–700 km) prior to convergence. Subduction primarily involved the slow and amagmatic subduction of a narrow domain of dry lithospheric mantle. This type of congested Ampferer subduction led to the sequential and coherent accretion of inherited rifted domains which today form the Alpine and Pyrenean orogens.

Modelovanje zatvaranja uskih okeanskih basena numeričkim simulacijama

Numerical simulations represent an established approach in investigation of geodynamic processes in the Earth's interior and it is applied to a vast variety of geological problems. The closure of a narrow oceanic realm is modelled in this paper. Firstly, a simplified numerical model of a narrow ocean is established. The thermomechanical system is then simulated by solving Navier-Stokes and temperature equations. By imposing boundary conditions for velocity the convergent regime is simulated which leads to a subduction process along a predefined weak shear zone mantle. The subduction subsequently results in development of magmatism, the final closure of the oceanic realm and the onset of continental collision. Considerable amount of ophiolites were successfully obducted onto the overriding continent during the course of the simulation.

Neo‐Tethyanslab tearing constrained by PalaeoceneN‐MORB‐like magmatism in southern Tibet

Southern Tibet is a key region for investigating the magmatism that occurredduring the collision between India and Asia. The break‐off of the Neo‐Tethyan slab, after the onset of continental collision at ca. 65–60 Ma, affected the lithospheric thermal regime and led to a series of magmatic and tectonic events. Many magmatic rocks generated by slab break‐off have been reported in southern Tibet; however, which of these record the initial slab break‐off (i.e., slab tearing) is still an open question. The Lianxiang gabbros, which intrude an ophiolite in central‐southern Tibet, provide an ideal opportunity to investigate the footprint of the initial slab break‐off. Zircon U–Pb ages show that the Lianxiang gabbros crystallized at 57–53 Ma, coeval with other mafic dikes in southern Tibet. The Lianxiang gabbros are characterized by high Na2O (2.13–5.42 wt%), Al2O3(13.86–17.13 wt%), and V (188–346 ppm) contents and moderate Mg# (40.8–65.9). The gabbrosare slightly depleted in light rare earth elements [LREE; (La/Yb)N= 0.45–0.76], similar to normal mid‐ocean ridge basalt (N‐MORB). They also have low87Sr/86Sriratios (0.703538–0.705698) and highεNd(t) values (9.49–10.68), indicating that they were derived mainly from a shallow asthenospheric mantle source. Combining our results with data from other magmatism at ~57 Ma in the southern Lhasa Terrane, we propose that the N‐MORB‐type rocks reflect the tearing of the Neo‐Tethyan slab. The process of slab break‐off differed between western and eastern southern Tibet, possibly as a result of different lithospheric structures and compositions.

Preorogenic Folds and Syn‐Orogenic Basement Tilts in an Inverted Hyperextended Margin: The Northern Pyrenees Case Study

AbstractThe Chaînons Béarnais (CB, North Pyrenean Zone) resulted from the Cenozoic contractional reactivation of the salt tectonics‐bearing, hyperextended margin that initiated at the Europe‐Iberia transition during the Early Cretaceous. In this tectonic scenario, assessing the relative contribution of extension and contraction to the present‐day structure is crucial to reconstruct the hyperextended margin geometry and to quantify the subsequent shortening. This study undertakes this issue by defining the relationship between folding and two bedding‐independent references: peak temperature isotherms and paleomagnetic data. Isotherms were reconstructed from 76 new measurements of Raman spectroscopy on carbonaceous materials (RSCM) and indicate temperatures at the time of peak metamorphism in the CB (110–85 Ma, end of extension). They are shallowly to moderately northwards dipping and cut across most of the folds deforming the Mesozoic units. Paleomagnetic data from 29 sites evidence a widespread remagnetization carried by pyrrhotite that was probably blocked during the early Paleogene (before the onset of continental collision) and postdated folding in the CB. Abnormal inclinations in this remagnetization suggest syn‐collision tilts up to 60° to the north in the back limb of the Axial Zone. Based on the presented data set, we propose that the folding of the cover above the evaporitic décollement was almost fully completed by the end of the Cretaceous extension, with ~85–100% of the dip of fold limbs being acquired before the remagnetization time. Cenozoic contraction reactivated the extensional faults in the shallow basement as top‐to‐the‐S thrusts, leading to the passive transport and northwards tilting of the folded cover.

Western Mediterranean Subcontinental Mantle Emplacement by Continental Margin Obduction

AbstractObduction is the tectonic process that results from thrusting of an oceanic lithosphere section (ophiolite) onto a continent. In contrast, thrusting of subcontinental mantle, observed in several mountain belts, remains a major unknown of plate tectonics. In the western Mediterranean, the Ronda and Beni Bousera peridotites are the largest worldwide subcontinental mantle exposure. From a geological point of view, the Ronda peridotites are exhumed by hyperstretching of the continental lithosphere in a back‐arc immediately followed by thrusting, explaining their present‐day position inside the Alboran crust. Using 2‐D and 3‐D modeling of new gravimetric data combined with local seismic tomography, we show that the Ronda peridotites are rooted inside the Alboran mantle along the entire Gibraltar arc. On these bases, we propose that the emplacement of the Ronda peridotites occurred in a back‐arc setting and corresponds to the thrusting of an entire hyper‐stretched continental margin onto a continent, a process that we define as continental margin obduction. This results from two successive deformation events: continental upper plate extension driven by slab roll‐back, immediately followed by upper plate shortening, likely triggered when a buoyant continental domain enters the subduction. We propose that this process affected the entire western Alboran domain, with peridotite bodies embedded within the crust along the whole Gibraltar Arc. We suggest that other examples such as the Alps Ivrea mantle body could likely represent continental margin obduction at the onset of continental collision.

Metamorphic Zonation by Out‐of‐Sequence Thrusting at Back‐Stepping Subduction Zones: Sequential Accretion of the Caledonian Internides, Central Sweden

AbstractExhumation of the high‐grade metamorphic Seve Nappe Complex and its emplacement between lower‐grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jämtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top‐to‐the‐ESE, foreland‐directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation‐parallel boudinage, and abundant top‐to‐the‐ESE and rare, scattered top‐to‐the‐WNW shear‐sense indicators imply foreland‐directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in‐ and out‐of‐sequence thrusts that generated the metamorphic zonation. Our model envisions in‐sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out‐of‐sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high‐grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica.

Syn-orogenic magmatism over 100 m.y. in high crustal levels of the central Grenville Province: Characteristics, age and tectonic significance

The Escoumins Supracrustal Belt (ESB) represents higher levels of the infrastructure of a large hot orogen, exposed in a broadly dome and basin pattern. It consists of remnants of a Pinwarian-age (1.52–1.46 Ga) oceanic arc and arc-rift sequence, preserved in the low-P Belt of the central Grenville Province, and was intruded by diverse Grenvillian-age plutons. The plutonic rocks range from quartz monzodiorite to granite and have intrusion ages covering a time interval of ~100 My, that represents the entire range of the Grenvillian orogeny. Moreover, the ages, field relations and geochemical signatures of the different intrusions can be matched with different documented stages of the orogeny. The oldest pluton, the magnesian, biotite-bearing Bon-Désir granite (1086 ± 2 Ma), has positive εNd (+0.6), TDM = 1.52 Ga, and is attributed to melting of a juvenile Pinwarian crust as a result of slab break-off, at the onset of continental collision. The ferroan and Ba–Sr enriched, biotite-, amphibole- and clinopyroxene-bearing Michaud plutonic suite (1063 ± 3 Ma) and biotite-rich felsic sill (1045 ± 3 Ma) have εNd (−0.01 − +0.8) and TDM = 1.45–1.48 Ga. Their geochemistry is consistent with fractionation of a mafic magma derived from melting of a Geon 14 subduction-modified subcontinental lithospheric mantle. This magmatism is consistent with convective thinning of subcontinental lithosphere, potentially linked to tectonic extrusion and orogenic collapse. This collapse ultimately led to the juxtaposition of the low-P Belt with the high-T mid-P Belt in the hinterland of the Grenville Province and to amphibolite-facies metamorphism in the former, producing metamorphic zircon overgrowths at 1037 ± 10 Ma. Finally, 988 ± 5 Ma to 983 ± 5 Ma syn-kinematic peraluminous two-mica garnetiferous leucogranite bodies and pegmatites with inherited 1055 ± 2 Ma metamorphic monazite were derived from melting of previously metamorphosed deeper levels of the low-P Belt. This is consistent with a high geothermal gradient linked to thinning of the crust in a Basin and Range setting. The geochemical and age pattern of Grenvillian-age magmatism in the ESB, in conjunction with the overall architecture of the Province, suggests that Laurentia was the upper plate during the Grenvillian orogeny.

Mantle Lithosphere Rheology, Vertical Tectonics, and the Exhumation of (U)HP Rocks

AbstractNumerical modeling results indicate that mantle lithosphere rheology can influence the pressure‐temperature‐time (P‐T‐t) trajectories of continental crust subducted and exhumed during the onset of continental collision. Exhumation of ultrahigh‐pressure (~35 kbar)/high‐temperature (~750°C) metamorphic rocks is more prevalent in models with stronger continental mantle lithosphere (e.g., dry), whereas high‐pressure (~9–22 kbar)/low‐temperature (350°C–630°C) metamorphic rocks occur in models with weaker rheology (e.g., hydrated) for the same layer. In the latter case, the buried crustal rocks can remain encased in ablatively subducting mantle lithosphere, reach only moderate temperatures, and exhume by dripping/detachment of the lithospheric root. In this transition from subduction to a dripping style of “vertical tectonics,” burial and exhumation of crustal rocks are driven without imposed far‐field plate convergence. The model results are compared against thermobarometric P‐T estimates from major (ultra)high‐pressure metamorphic terranes. We propose that the exhumation of high‐pressure/low‐temperature metamorphic rocks in Tavşanlı and Afyon zones in western Anatolia may be caused by viscous dripping of mantle lithosphere suggesting a weaker continental mantle lithosphere, whereas (ultra)high‐pressure exhumation (e.g., Dabie Shan‐eastern China and Dora Maira‐western Alps) may be associated with plate‐like subduction. In the latter case, the slab is much stronger and deformation is localized to the subduction interface along which rocks are buried to >100 km depth before they are exhumed to the near surface.

Numerical models of the magmatic processes induced by slab breakoff

After the onset of continental collision, magmatism often persists for tens of millions of years, albeit with a different composition, in reduced volumes, and with a more episodic nature and more widespread spatial distribution, compared to normal arc magmatism. Kinematic modelling studies have suggested that slab breakoff can account for this post-collisional magmatism through the formation of a slab window and subsequent heating of the overriding plate and decompression melting of upwelling asthenosphere, particularly if breakoff occurs at depths shallower than the overriding plate.To constrain the nature of any melting and the geodynamic conditions required, we numerically model the collision of two continental plates following a period of oceanic subduction. A thermodynamic database is used to determine the (de)hydration reactions and occurrence of melt throughout this process. We investigate melting conditions within a parameter space designed to generate a wide range of breakoff depths, timings and collisional styles.Under most circumstances, slab breakoff occurs deeper than the depth extent of the overriding plate; too deep to generate any decompressional melting of dry upwelling asthenosphere or thermal perturbation within the overriding plate. Even if slab breakoff is very shallow, the hot mantle inflow into the slab window is not sustained long enough to sufficiently heat the hydrated overriding plate to cause significant magmatism. Instead, for relatively fast, shallow breakoff we observe melting of asthenosphere above the detached slab through the release of water from the tip of the heating detached slab. Melting of the subducted continental crust during necking and breakoff is a more common feature and may be a more reliable indicator of the occurrence of breakoff. We suggest that magmatism from slab breakoff alone is unable to explain several of the characteristics of post-collisional magmatism, and that additional geodynamical processes need to be considered when interpreting magmatic observations.

Kinematics of syn- and post-exhumational shear zones at Lago di Cignana (Western Alps, Italy): constraints on the exhumation of Zermatt–Saas (ultra)high-pressure rocks and deformation along the Combin Fault and Dent Blanche Basal Thrust

Kinematic analyses of shear zones at Lago di Cignana in the Italian Western Alps were used to constrain the structural evolution of units from the Piemont–Ligurian oceanic realm (Zermatt–Saas and Combin zones) and the Adriatic continental margin (Dent Blanche nappe) during Palaeogene syn- and post-exhumational deformation. Exhumation of Zermatt–Saas (U)HP rocks to approximately lower crustal levels at ca. 39 Ma occurred during normal-sense top-(S)E shearing under epidote–amphibolite-facies conditions. Juxtaposition with the overlying Combin zone along the Combin Fault at mid-crustal levels occurred during greenschist-facies normal-sense top-SE shearing at ca. 38 Ma. The scarcity of top-SE kinematic indicators in the hanging wall of the Combin Fault probably resulted from strain localization along the uppermost Zermatt–Saas zone and obliteration by subsequent deformation. A phase of dominant pure shear deformation around 35 Ma affected units in the direct footwall and hanging wall of the Combin Fault. It is interpreted to reflect NW–SE crustal elongation during updoming of the nappe stack as a result of underthrusting of European continental margin units and the onset of continental collision. This phase was partly accompanied and followed by ductile bulk top-NW shearing, especially at higher structural levels, which transitioned into semi-ductile to brittle normal-sense top-NW deformation due to Vanzone phase folding from ca. 32 Ma onwards. Our structural observations suggest that syn-exhumational deformation is partly preserved within units and shear zones exposed at Lago di Cignana but also that the Combin Fault and Dent Blanche Basal Thrust experienced significant post-exhumational deformation reworking and overprinting earlier structures.

Syn-collisional granitoids in the Qilian Block on the Northern Tibetan Plateau: A long-lasting magmatism since continental collision through slab steepening

In this paper we present a new model that can explain the large zircon age spectrum of ~510 – 420Ma within a single sample from the Gangcha (Gcha) biotite granodiorite and the Huangyuan (HY) two-mica monzogranite on the northern Tibetan Plateau. The large age spread recorded in zircons is characteristic of granitoid samples from the studied region, which is best explained by the long-lasting magmatism since the onset of continental collision at ~500Ma, followed by slab steepening and the ultimate slab break-off at ~450Ma. These granitoids have a large major and trace element compositional variation, but limited initial Sr (ISr[450]=0.709 to 0.715), Nd (ԐNd[450]=−6.5 to −3.7), Hf (ԐHf[450]=−4.3 to 1.5) and Pb (206Pb/204Pb[450]=17.70 to 17.17; 207Pb/204Pb[450]=15.60 to 15.69; 208Pb/204Pb[450]=38.04 to 38.73) isotopic variation. The small negative whole rock ԐNd[450] and ԐHf[450] values are most consistent with the granitoid source being dominated by subducted seafloor materials. The inherited zircons with large negative ԐHf[450] values (e.g. −50) are indicative of input from the lower continental crust and subducted sediments. The correlated variations among major elements, trace elements and radiogenic isotopes are best interpreted as reflecting melting-induced mixing of a compositionally heterogeneous source with superimposed effect of varying extent of fractional crystallization and crustal assimilation. The inherited zircons of Palaeo-Proterozoic age and the Archean crustal model ages signify the involvement of ancient basement rocks.

Cenozoic exhumation of the internal Zagros: first constraints from low-temperature thermochronology and implications for the build-up of the Iranian plateau

The Iranian plateau is a flat ~ 1.5–2 km high plateau thought to result from the collision between the Arabian and Eurasian plates since ~ 30 ± 5 Ma, and may represent a young analogue to the so far better studied Tibetan plateau. In order to constrain the exhumation history of the internal Zagros and of the Iranian plateau, we herein present apatite (U-Th)/He (AHe) and apatite (AFT) and zircon fission-track (ZFT) data on plutonic rocks from the Sanandaj–Sirjan Zone (SSZ), Urumieh–Dokhtar magmatic arc (UDMA), Central Iran and Kopet Dagh. Thermochronologic data show that the SSZ was exhumed early in the collision process (essentially before 25–20 Ma), with a likely acceleration of cooling during the late Eocene, from 0.04 to 0.3 mm/year. Results suggest that cooling of the internal Zagros migrated from the SSZ to the UDMA during a more mature stage of the continental collision, after ~ 17 Ma (i.e., coeval with the outward propagation of deformation and topography fronts in the external Zagros). Constant exhumation rates in the UDMA (~ 0.3 mm/year) suggest that no significant variation of erosion rates occurred since the onset of continental collision. In Central Iran, the overlap of ZFT, AFT and AHe ages from gneissic samples points to rapid cooling during the late Eocene (~ 42 °C/Myr), which is consistent with previous reports on the formation of Eocene metamorphic core-complexes.

Tectonic accretion and recycling of the continental lithosphere during the Alpine orogeny along the Pyrenees

Abstract The goal of this paper is to identify the fate of the continental lithosphere along the Iberia-Eurasia convergent plate boundary marked by the formation of the Pyrenean orogenic belt. The present-day volumes of crust and lithosphere beneath the Pyrenees and the volume of eroded crust redistributed in neighbo ring basins are evaluated based on a synthesis of available geological and geophysical data. The volumes that are expected to have transited across the former plate boundary are modeled taking into account Iberia-Eurasia convergence and making assumptions regarding the initial lithospheric and crustal structure of the Iberia-Eurasia plate boundary at the onset of continental collision (~83 Ma). Despite large uncertainties, the difference between the initial and present-day lithospheric structures suggests that at 83 Ma, either the Iberia-Eurasia plate boundary was marked by a zone of thinned lithosphere (oceanic and/or continental), or the lithosphere having transited across the plate boundary has for the most part been recycled into the mantle. At the crustal-scale, the volume of tectonically accreted crust is estimated by adding the volume of crust currently present in the Pyrenean orogenic belt to the volume of sediments deposited in neighboring basins, and by subtracting the initial volume of crust at the onset of continental collision considering two end-members, namely (i) a continental rift or (ii) a 35 km wide oceanic basin. In both cases, this tectonically accreted crustal volume is not enough to match the calculated volume of crust that has potentially transited across the plate boundary as a consequence of convergence since 83 Ma. As a result, our computation suggests that at least 30% (and as much as 63%) of the continental crust has subducted with the Iberian lithospheric slab and has been recycled into the mantle. In addition, the synthesis of topographic and geophysical (gravity and seismic tomography) reveals a peculiar crustal and lithospheric scale structure for the current day Pyrenees characterized by (i) an elliptical-cone-shape Pyrenean mountain range underlain by an elliptical-cone-shaped crustal root pointing down, and (ii) two tongues of lithospheric mantle in the central part of the belt. These features are interpreted as reflecting redistribution of the lithospheric mantle and of the orogenic crust by ductile flow after subduction and tectonic accretion. We propose that following a period of subduction/collision from 83 to 35 Ma, the decrease in the convergence rate between Iberia-Eurasia favored thermal relaxation of the Iberian slab promoting ductile flow and the development of gravitational instabilities. We suggest that the orogenic root has been dragged down by the dense lithospheric root and that part of it has been recycled into the mantle. In this view, the current-day lithospheric tongues represent the remnants of the lithospheric root after thermal relaxation and recycling by convective removal.

Pliocene–Quaternary volcanic rocks of NW Armenia: Magmatism and lithospheric dynamics within an active orogenic plateau

The Pliocene–Quaternary volcanic rocks of Armenia are a key component of the Arabia–Eurasia collision, representing intense magmatism within the Turkish–Iranian plateau, tens of millions of years after the onset of continental collision. Here we present whole rock elemental and NdSr isotope data from mafic, intermediate, and felsic lava flows and cinder cones in Shirak and Lori provinces, NW Armenia. Magmatism appears to be controlled locally by extension related to major strike-slip faults within the plateau. Major and trace element results show that the three series—valley-filling medium-K alkali basalt flows, ridge-forming andesite to rhyolite flows, and andesitic cinder cones—form a compositional continuum linked by a crystallisation sequence dominated by two pyroxenes, plagioclase and amphibole. There is petrographic and major and trace element evidence for magma mixing processes and potentially crustal contamination by Mesozoic–early Cenozoic arc-related rocks, which has not significantly affected the isotopic signature. Modelling of the basaltic rocks indicates that they formed by moderate degrees of partial melting (~3–4%) of an incompatible element enriched, subduction-modified, lithospheric mantle source. Samples have a distinctive high Zr/Hf ratio and high Zr concentrations, which are an intrinsic part of the source or the melting process, and are much more commonly found in ocean island basalts. Regional models for magmatism often argue for whole-scale delamination of the mantle lithosphere beneath Eastern Anatolia and the Lesser Caucasus, but this scenario is hard to reconcile with limited crustal signatures and the apparent lack of asthenospheric components within many studied centres.

Kinematic evolution of Alpine Corsica in the framework of Mediterranean mountain belts

The tectonic evolution of Alpine Corsica and surrounding areas is described through a series of plate reconstructions from the onset of the Alpine convergence in the early Cretaceous (~120Ma) to the early Miocene (~19Ma). The southward subduction of Eurasian lithosphere (Alpine Tethys) beneath Adria and Iberia, and the subduction of Ligure–Piemontese Ocean (Iberian plate) beneath the Adria margin, led to the formation of a trench–trench–trench triple junction close to the northern margin of Adria. During the first phase, from the early Aptian (~120Ma) to the Campanian (~75Ma) the northeastern oceanic part of the Iberian plate subducted beneath Adria along an intra‐oceanic subduction zone between Corsica and Adria. This trench was linked to the eastern Iberian convergent margin (having opposite polarity) through a transform fault. Starting from the Campanian and until the Oligocene (~34Ma) oceanic and transitional crust was obducted onto Hercynian Corsica and Eurasia, giving rise to HP metamorphism within the subducted continental crust. From ~40Ma to the early Oligocene (~33Ma) the onset of continental collision in the Alpine area led to thickening of the subducting Eurasian margin through the formation of the European lower crustal wedge. Finally, from the early Oligocene (~33Ma) to the early Miocene (~19Ma), during the rotation of the Corsica and Sardinia blocks, the Pyrenean–Alpine belt lost its continuity in Gulf of Lyon and Provençal area, while the western Adriatic margin (Apennine domain) became a left‐lateral transpressive belt linking the western Alpine chain with the Calabrian Arc.

Continental collision and slab break-off: A comparison of 3-D numerical models with observations

Conditions and dynamics of subduction–collision and subsequent 3-D slab break-off and slab tear propagation are quantified, for the first time, using fully dynamic numerical models. Model results indicate that collision after the subduction of old, strong subducting oceanic slab leads to slab break-off at 20–25Myr after the onset of continental collision, and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100–150mm/yr. In contrast, young, weak oceanic slabs show the first break-off already 10Myr after continental collision, and can experience tear migration rates up to 800mm/yr. Slab strength plays a more important role in the timing of slab break-off and the speed of a propagating slab tear than (negative) slab buoyancy does. Slab break-off is viable even for slabs that are supported by the viscosity jump and phase change between the upper and lower mantle.The density of the oceanic slab and the subducting continental block is important for the amount of continental subduction and the depth of slab break-off. A 40-km thick continental crust can be buried to depths greater than 200km, although this maximum depth is significantly less for younger or very weak slabs, or thicker continental crust. Slab break-off typically starts at a depth of 300km, mostly independent of mantle rheology, but, like continental crustal burial, can be shallower for young or buoyant plates. Our 3-D models illustrate how, due to the difference in necking in 2-D and 3-D, break-off has an intrinsic small preference to start as a slab window within the slab's interior, rather than as a slab tear at the slab edge. However, any significant asymmetry in the collision setting, e.g. earlier collision at one end of the subduction zone, would override this, and leads to slab tearing starting near one edge of the slab.These results put important new constraints on the dynamics of the collision and subsequent slab break-off for modern collision belts. For a proposed timing of the initial Arabia–Eurasia collision at 35Ma, break-off of the ~200-Myr-old Neo-Tethys slab is unlikely to have occurred before 15–10Ma. Furthermore, our results illustrate that shallow, early break-off of weak slabs provides a viable explanation for the absence of blueschists and ultra-high pressure metamorphism in the Precambrian geological record.

Detrital Modes in a Late Miocene Wedge-Top Basin, Northeastern Calabria, Italy: Compositional Record of Wedge-Top Partitioning

Abstract The composition and stratigraphic relations of sandstones and associated conglomerates derived from erosion of the northern Calabrian terranes provide important constraints for paleotectonic and paleogeographic models of the Neogene wedge-top depozone evolution of the southern Italy foreland basin system. After onset of continental collision in the southern Italy orogenic belt during late Oligocene to early Miocene, the Calabrian Arc drifted northeastward, accreting over the Adria lithosphere during mid to late Miocene time. Since the Tortonian, the northern Calabrian terranes represented an uplifted thrust belt in which along its leading edge tectonic subsidence was responsible for accumulation of a thick Tortonian to Quaternary succession, dominantly siliciclastic (up to 3300 meters thick), deposited in the wedge-top depozones (Rossano and Crotone basins). Sandstone assemblages have a homogeneous quartzofeldspathic composition in the Rossano and Crotone basins, characterized by a compositional trend, through time in response to regional tectonics. Evolution of the basins fill occurred during (1) Serravallian?–Tortonian, (2) late Tortonian to early Messinian, and (3) late Messinian. Detritus shed into those basins was derived mainly from terranes within the strongly deformed Sila Mountains allochthon, which was one of the principal structural elements of the nearby northern Calabrian terranes, consisting of Hercynian plutons intruding Paleozoic (Cambrian to Carboniferous) metasedimentary host rocks, and locally having a sedimentary cover of Mesozoic (Longobucco Group) or early Miocene (Paludi unit) clastics. Low- to medium-grade metamorphic and minor plutonic grain types (metamorphiclastic petrofacies) represent the main detritus in the earliest (Serravallian?–Tortonian) sedimentation that accompanied progressive flexural bending of the western margin of the foreland-basin system. Tortonian to early Messinian time was characterized by isolation of the Mediterranean region from the global marine realm, leading to evaporite sedimentation in the foreland in addition to intense regional and local tectonics. Compositional response to those events was a marked increase in sedimentary detritus (sedimenticlastic petrofacies) related to cannibalization of early Messinian evaporite successions; after the evaporitic events within the basins and the gradual re-establishment to the normal marine conditions, sandstone detrital modes are characterized mainly by a return of the Serravallian?–Tortonian compositional modes (i.e., metamorphic–plutoniclastic petrofacies). The post-evaporitic sedimentation corresponds to a major tectonic rearrangement on both Rossano and Crotone basins related to extrusion and inversion of the basal portion of the Serravallian?–Tortonian sequences in the intermediate zone to form an intervening Ciro structural high. In the Crotone Basin during late Messinian, deposition of the Carvane Conglomerate records an abrupt sudden arrival of distinctive sedimentary containing clasts of detritus of reddish limestone, marl, quartz arenite, siltstone and chert derived from Mesozoic to Tertiary basinal sequences of the southern Apennines domains (i.e., Sicilide Complex terranes). Detrital-mode evolution of the inner portions of the late Miocene southern Italy foreland-basin system was in response to abrupt tectonic partitioning due to continuation of accretionary processes, rapid uplift of mid-crustal blocks, and the superposition of wrench tectonics.

The age of the oceanic accretionary wedge and onset of continental collision in the Sicilian Maghrebian Chain

New biostratigraphic data from the formations unconformably lying above the tectonic units resulting from the Flysch Basin Domain (FBD) in the Sicilian Maghrebids are here reported. The FBD constituted a southern branch of the western Tethys, separating during the Jurassic to Paleogene a Mesomediterranean Microplate from the African Plate. The age of the youngest sediments involved in the nappes and that of the unconformable terrains deposited in thrust-top basins on these tectonic units, allow to define both the age of deformation of the oceanic realm and the onset of the continental collision. The deformation migrated from internal to external areas of FBD starting from the latest Burdigalian-Langhian to Serravallian. Therefore, the previously proposed Eocene-Oligocene mesoalpine deformation of the Maghrebian FBD cannot be supported. The continental collision started during the Serravallian and it was accomplished in the late Tortonian, when clastic deposits sealed the boundaries of the tectonic units originated from all the paleogeographic domains of the Maghrebian Chain. The steps of the tectonic evolution in the Sicilian Maghrebids are now very well constrained and the proposed tectonic evolution may be extended to the whole Maghrebian Chain, as far as to the western Betic Cordilleras and to the southern Apennines, where most of the tectonic events show highly similar features and ages.